Blue Harbour Group LP has built a roughly 6.2% position in Adient PLC, making the activist investor one of the automotive-seat supplier’s biggest shareholders less than a year after it was spun into a stand-alone company.

Blue Harbour’s investment, its biggest bet ever at about $410 million, is based on a thesis that Adient can dramatically improve its margins, boost share buybacks and rework its network of joint ventures in China, Blue Harbour partner Peter Carlin said in an interview. Mr. Carlin believes management is on that path and supports the work it is doing, a typical stance for Blue Harbour, which is known as a friendly activist. Still, Mr. Carlin believes the market is missing value in the company.

Representatives for Adient didn’t immediately respond to requests for comment.

In making a bet on the company spun out of Johnson Controls International PLC last year, Blue Harbour is the latest activist to seek value lurking in a recent spinoff rather than just letting the act of spinning the company off create value.

That strategy has worked for Blue Harbour. The new investment comes on the heels of the firm exiting its most profitable investment ever, a stake in BWX Technologies Inc., another company that had just gone through a breakup. The investment firm attributes its profits almost entirely to gains made after that split, a sort of blueprint for Adient.

Blue Harbour in 2014 bought into what was then Babcock & Wilcox Co. and called for a split of the company’s utility business from the one building nuclear reactors for the Navy. When that happened on July 1, 2015, Blue Harbour stuck with the naval systems company, BWX, and partner Robb LeMasters joined the board. The fund has now sold out of the stake, and it made more than $250 million of profit on a position it paid about $240 million for, according to filings.

That is the $3.5 billion firm’s biggest profit on any single investment since Clifton S. Robbins, a former KKR & Co. partner, started the fund in 2004, aiming to take sizable stakes and work with management groups.

Mr. LeMasters credits the gains at BWX to moves he said were enabled by the separation, which simplified the business, allowing it to concentrate on where to spend capital and buy back more stock.

Blue Harbour disclosed a roughly 3.7% stake in Adient in an August filing, but it has ramped up its buying in recent weeks. A 6.2% stake would make it the company’s third-biggest holder, according to FactSet data. Adient has a 33% market share in global automotive seats, and it services all the major car makers around the globe.

Adient has already been making a push to improve its profit margins, and Blue Harbour thinks there are even more increases coming, Mr. Carlin said.

Meanwhile, the stock trades at lower multiples than rivals, so Blue Harbour is urging management to ramp up its share repurchases. Adient initiated share repurchases and a dividend in the fiscal third quarter ended in June.

Blue Harbour also said the complexity of Adient’s Chinese business, which is based on 17 joint ventures with car manufacturers there, making for complicated financial statements, has hurt its stock value. The activist thinks that obscures what it believes are large cash positions at the JVs. China’s car market, and Adient’s strong position there, are reasons why Blue Harbour likes the stock. The value could be clarified by consolidating the joint ventures, or even by better educating investors on how Adient makes money there, Blue Harbour believes.

Blue Harbour recently told investors it is focused heavily on social and governance matters as a financial risk, and pored over Adient’s Chinese businesses to look for any red flags. It didn’t find any.

Write to David Benoit at david.benoit@wsj.com

Breaking the story

David Benoit was ahead of competitors with the news that Blue Harbour Group LP had acquired a roughly 6.2% stake in auto supplier Adient PLC, making the activist investor one of Adient’s biggest shareholders less than a year after it was spun into a stand-alone company.

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